1. Field of the Invention
This invention relates to thermoplastic compositions and is more particularly concerned with thermoplastic polymer blends comprising predominantly polyurethane elastomers and fire retardant compositions derived therefrom.
2. Description of the Prior Art
Many types of blends of thermoplastic polyurethane polymers with other thermoplastic polymers have been utilized to overcome some deficiency in properties in either the polyurethane itself or in the other ingredient(s).
For example, the low temperature properties of vinyl chloride polymers (PVC) can be improved by blending them with polyurethanes as typically disclosed in U.S. Pat. No. 3,257,261. Multicomponent blends like those derived from polyurethanes, PVC resins, and chlorinated polyethylene (CPE) disclosed in U.S. Pat. No. 3,882,191 have been employed to improve processing characteristics while at the same time providing economically attractive products. Other similar blends are disclosed in U.S. Pat. Nos. 3,381,056 and 4,111,894.
U.S. Pat. No. 3,970,716 discloses fire retardant blends consisting of thermoplastic polyurethanes, chlorinated polyethylenes, and a narrow group of chlorine containing fire retardant materials. The disclosed blends are characterized by the highest self extinguishing (SE) rating of SE-O when tested by the Underwriters Laboratory (UL) test designated UL 94 (test defined below). However, chlorinated polyethylenes do not stand up well to the high processing temperatures met in some injection molding procedures. This results in the blend tending to degrade with overall loss in polymer properties.
British Pat. No. 1,101,665 discloses adhesive compositions of organic solutions consisting of polyesterurethanes, butadiene acrylonitrile copolymers and vinyl chloride-vinyl acetate copolymers.
U.S. Pat. No. 3,678,129 discloses thermoplastic polymer blends of predominantly polyvinyl chloride mixed with minor amounts of a polyether polyurethane and butadiene-acrylonitrile copolymers. The latter two ingredients give rise to improved abrasion resistance of the polyvinyl chloride.
We have now discovered that blends of thermoplastic polyurethanes with polyvinyl halide resins and diene-nitrile copolymer rubbers in proportions heretofore not taught by the prior art provide thermoplastic polymers having unique and unexpected char forming properties when the blends are subjected to flame tests. This ready formation of a char upon combustion has not hitherto been observed for polyurethane polymers alone.
In contrast to pure PVC and blends rich in PVC which burn with the evolution of much dense black smoke, including those PVC blends described in U.S. Pat. No. 3,678,129 cited supra, the present blends do not evolve dense black smoke when forming the char.
Furthermore, standard fire retardant additives can be added into the present blends to impart increased resistance to burning including fire retardants of the type which have been taught by U.S. Pat. No. 3,970,716 as not being useful in preventing flaming drip of polyurethanes. Also the present blends do not suffer any of the high temperature processing problems referred to above for the prior art blends disclosed in U.S. Pat. No. 3,970,716.
Additionally, the physical properties of the present polymer blends including those with the fire retardant additives retain, to a major extent, the good properties of the original polyurethanes.